I was brought up around a lot of judgement, and it was so passive and pervasive. It was embedded in my brain before I could stand back and look at myself. My dad could be termed a "people-watcher" by today's standards. Whether it was people we knew, or just someone walking down the street, he would have commentary on their looks, how they acted, or what they said. I'll admit that I still find myself judging people, but not nearly as much.
Maybe things like that had a deal of influence of why I started lifting. All I know is that insecurity is not a big motivating factor now on why I lift. And yes, I do lift for "aesthetics" primarily but health is important too. Personally, lifting is a hobby now. Just like reading or making music. The closest comparison to why I lift would maybe be someone who likes getting tattoo's or piercings. Seeing my body change is awesome. Then again, I guess the insecurity argument can be made for people who like tattoos or piercings too.
I think insecurity is a big contributing factor for why a lot of people lift, use steroids, etc. Not everyone, though.